1. Field of the Invention
The present invention is related to storage systems within computer systems, and more specifically to a hybrid storage subsystem including multiple device types in which file contents are spread across the device types.
2. Description of Related Art
In computer storage subsystems, multiple levels of storage hierarchy are often provided due to differing latencies in the storage devices themselves. Cache memories provide fast access to data for processors, while main system memory provides typically larger storage at longer latencies. Hard disk drive (HDD) systems, which are generally implemented using magnetic storage technologies, provide a backing store for main system memory that has an even longer latency and typically a greater capacity. However, file accesses to HDD drive systems are directed at the HDD level. The HDD level may have its own cache that caches a portion of a file, but the entire file is located on the HDD and all or a portion of the file is cached when accessed frequently, after the first access has occurred.
Recently, solid-state drives (SSDs) build from non-volatile semiconductor memories, have reach reliability levels, capacities and costs, such that their use as replacements for HDDs is becoming practical. Hybrid SSD/HDD storage systems have been proposed and implemented in which frequently-accessed files are stored on SSDs, while files for which access demands are less frequent are stored on HDDs. However, such a strategy does not take into account the dynamic nature of file system access frequency due to various applications and operating systems that may be using a storage system over any given period.
Other techniques have been employed involve dynamic migration of disk contents based upon a least-recently-used (LRU) or other scheme that determines the relative access frequency of files and tables in order to migrate less frequently used files downward in the hierarchy, i.e., from the SSD portion of the disk/array to the HDD portion. Such a scheme improves efficiency, but large amounts of space may be wasted in the SSD if only a small portion of the file(s) remaining on the SSD are actually being accessed frequently. For example, a large database file may be the most frequently accessed file in a system, but only a small portion of the database may actually be undergoing access over a significant time interval.
A technique that could avoid the above-described problem is to use the SSD portion of the disk/array as a software cache. However, such an implementation introduces another layer of complexity to operating system software and another level of access latency into the system.
Therefore, it would be desirable to provide a management method and hybrid disk storage system that provides low average access latency along with low complexity.